Monoclonal antibodies were used to characterize and isolate antigens of the rodent malaria parasite Plasmodium yoelii. Those antigens restricted to the merozoite stage, which might be involved in cell penetration, were examined in detail. Studies showed that at least two antigens were present in all Sporozoa tested which possessed rhoptries and micronemes, but were absent in all parasites that lack these structures. New studies were initiated to follow the kinetics of antimalarial antibody formation during the course of self-limiting malaria using a quantitative isotype specific solid-phase radioimmunoassay (RIA). The appearance of protective antimalarial antibodies was compared with the amount of antibody present in the serum and the number of malarial antigens which induced the response. Protective antibodies were found to be present primarily in the IgG2a fraction of hyperimmune serum.